In the process of manufacturing silicon or other semiconductor microchips, light is generally directed through a reticle mask to etch circuits into a silicon wafer. The presence of dirt, dust, smudges or other foreign matter on the surfaces of the reticle mask or the silicon wafer is highly undesirable and adversely affects the resulting circuits. As a result, the reticles and silicon wafers are necessarily inspected before use. One common inspection technique is for a human inspector to visually examine each surface under intense light and magnification. Debris that is smaller than can be visually detected, however, impairs the resulting microchips.
Laser inspection systems have been developed for inspecting the surface of silicon wafers to accurately detect small particles. Examples of such systems may be seen in U.S. Pat. No. 4,197,011 by Hudson entitled "Defect Detection And Plotting System"; U.S. Pat. No. 4,376,583 by Alford et al. entitled "Surface Inspection Scanning System"; U.S. Pat. No. 4,630,276 by Moran et al. entitled "Compact Laser Scanning System"; U.S. Patent; U.S. Pat. No. 4,643,569 by Sullivan entitled "Dual Beam Laser Inspection"; U.S. Pat. No. 4,875,780 by Moran, et al. entitled "Method and Apparatus For Inspecting Reticles"; U.S. Pat. No. 5,076,692 by Neukermans, et al. entitled "Particle Detection On A Patterned Or Bare Wafer Surface"; U.S. Pat. No. 5,127,726 by Moran entitled "Method And Apparatus for Low Angle High Resolution Surface Inspection"; and Japanese Patent 69,539 by Mochizuki entitled "Inspecting Device For Surface Defect."
In these traditional laser inspection systems, light is specularly reflected and scattered from a surface of an article. The specularly reflected and scattered light are both indicative of the presence of particles or flaws on the surface of the article. The light scattered from the reflective surface of the article, such as a silicon wafer disk, is relayed to a single photodetector such as a photomultiplier tube ("PMT"). In the traditional systems, the scattered light consists of a slowly varying base line signal sometimes referred to as haze. The signal typically also contains fast pulses of light which correspond to point defects, such as dust particles on the wafer. The photodetector responds to the base line signal with a direct current ("DC") level offset which is proportional to the haze level. Due to the statistical nature of electron amplification through the diode stages of the PMT, multifrequency alternating current ("AC") noise is generated. This AC noise is random in nature, or asynchronous, and limits capability to distinguish the desired particle signal from this "random" system noise.
Several laser inspection systems have been developed which recognize that the signals detected by various photodetectors do not provide clear indications of fine particles or flaws on the surface of the article. Examples of these systems may be seen in U.S. Pat. No. 4,464,050 by Kato et al. entitled "Apparatus for Detecting Optically Defects"; U.S. Pat. No. 4,861,164 by West entitled "Apparatus for Separating Specular From Diffuse Radiation"; and U.S. Pat. No. 4,893,932 by Knollenberg entitled "Surface Analysis And Method."These systems, however, failed to reduce the haze level so that detecting the signal from the haze is still difficult.
Thus, there is a need for a laser inspection system which increases signal-to-noise ratio ("S/N") from the scattered light channel detecting particles or flaws, in the surface of an article, such as a silicon wafer.